Herein, we report a detailed investigation on the influence of a series of tertiary alkylamine on the rate of the photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization under visible light in the presence and absence of oxygen using Eosin Y as a photo-organocatalyst. Most importantly, we discovered that a simple increase of the alkyl length substituents in the alkylamines leads to an enhanced rate of PET-RAFT polymerization and its tolerance to oxygen. This study contributes to the fundamental comprehension of PET-RAFT polymerization process and the role of the tertiary amine as a sacrificial electron donor agent.